In an electrophotographic method using an instrument such as a copying machine, a laser printer, and a plain-paper fax machine, printing is generally done via processes including: a charging process in which a photo-sensitive body is uniformly charged by using a charging equipment such as a charging roller; an exposure process in which a light is irradiated on the charged photo-sensitive body in accordance with a printed data thereby forming a static latent image; a development process in which the latent image is visualized by attaching a toner to the static latent image thus formed; a transcription process in which the toner attached to the latent image is transferred to a body to be printed such as a paper; and a fixing process in which the transferred toner is fixed by utilizing heat or the like.
In the printing method using an electrophotographic method like this, an image is formed by utilizing a static electricity which is generated by charging a toner frictionally; and thus, in order to obtain a high quality image and to increase a printing speed, not only the toner needs to be provided with a sufficient triboelectrostatic charge amount but also this triboelectrostatic charge amount provided to the toner needs to be stable. For this purpose, powders of a metal oxide such as silica, titania, and alumina have been conventionally added as an external additive to provide the toner with an intended charge amount, to stabilize the triboelectrostatic charge amount provided to the toner, to improve fluidity of the toner, and so forth. Especially, silica is used widely because silica having a very small particle diameter is available cheaply, excellent fluidity can be obtained when it is added to the toner, and charging amount thereof is so high that the toner can be provided with a sufficient charge.
However, a silica powder in itself is hydrophilic thereby easily influenced by a use environment such as temperature and humidity as well as by a printing environment; and thus, for example, there are such problems that the toner cannot be provided with sufficient fluidity under a high temperature and high humidity environment in which silica powders agglomerate, and that, under an environment in which a drastic humidity change takes place, a triboelectrostatic charge amount of the external additive itself is not stabilized thereby leading to insufficient charge control of the toner.
Because of this, for example, a silica powder whose surface is made hydrophobic by surface treatment is used as an external additive of a toner (for example, see Patent Document 1). In the invention according to Patent Document 1, a silane coupling agent and a silicone oil are used for surface treatment of a silica powder. As to the treating agent, use of a silicone varnish, a silylation agent, and the like are investigated.
On the other hand, if these treating agents used to afford the hydrophobicity are not attached or fixed adequately well to the powder surface, when this powder is added to a toner, a part thereof is readily released as passage of time or as the use frequency thereof increases. Because of this, in the powders having these treating agents insufficiently fixed thereto, the hydrophobic effect that is enhanced by the surface treatment decreases gradually; and as a result, resistance of the triboelectrostatic charge amount to humidity change decreases simultaneously. In addition, its fluidity-affording effect to the toner decreases too, thereby causing problems such as poor transcription in electrophotography, fouling of a machine, and so forth. To the problems like these, a toner which is added as the external additive with silica microparticles having a treating agent fixed thereto with the fixing rate of 30% or more by mass, or preferably 60% or more by mass, as the carbon base, is disclosed (for example, see Patent Document 2).